The invention relates to a frequency modulator and to a frequency modulation method.
A frequency modulator 101 as illustrated in FIG. 8 is known from the prior art.
This frequency modulator 101, which is implemented, for example, in the RKE transmitter families TDx51xx and TDA525x as well as PMA5100 of Infineon Technologies AG, Munich, comprises a crystal oscillator 102, a phase locked loop 106 and an electronic switch S which can be opened and closed in time with a modulation signal ME that is applied to an input EM.
An input of the crystal oscillator 102 is connected to ground via the series connection of a crystal Q to a series circuit comprising two capacitors C1, C2. The switch S is arranged parallel to one of these capacitors C1, C2 (namely C1) in such a manner that it bridges the latter when the switch S is closed.
In the case of a frequency modulator 101 of this type, a frequency-modulated signal which can be tapped off at an output A and can be emitted by an antenna 107 is frequency-modulated by frequency-modulating the reference frequency of the phase locked loop 106. The reference signal for the phase locked loop (PLL) 106 is provided by the crystal oscillator 102 which is frequency-modulated by changing the series capacitance CS of the crystal Q, said series capacitance being formed by the individual capacitances C1, C2.
When the switch S is open, the series capacitance CS=C1×C2/(C1+C2) is small, with the result that the oscillator frequency is high. When the switch S is closed, the series capacitance CS=C2 is large, with the result that the oscillator frequency is low.
The disadvantage of a frequency modulator of this type is that the data rate of the modulation signal is limited on account of transient processes when closing and opening the switch S.
Only small frequency shifts Δω are possible since the oscillation frequency of a crystal can be changed only to a slight extent. Since the oscillation frequency of the crystal oscillator must be designed to be variable in the case of a frequency modulator of the type described above, this results in the crystal oscillator having an associated low frequency stability. Finally, there is no shift flexibility whatsoever. In addition, no baseband filtering is possible.
The prior art also discloses using a system having direct digital synthesis (DDS) as the frequency modulator. Frequency modulators of this type are extremely complex and have only low efficiency. Only a frequency-modulated signal having a high parasitic oscillation component can be generated using currently available systems.
So-called IQ modulators are furthermore provided in the transmission path. However, IQ modulators of this type are likewise very complex and have low efficiency.